<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 后代选择器包含所有后代，儿子，孙子。。。 */
        div span {
            color: red;
        }

        .aa div {
            color: orange;
        }
    </style>
</head>
<body>
    <span>span 标签</span>
    <div class="aa">
        <span>这是div 的儿子</span>
        <p>
            <span>
                孙子 span
            </span>
        </p>
        <div>aaa</div>
    </div>
</body>
</html>